Superheterodyne radio receiver having compensating means for frequency drift of the received carrier wave



Dec. 30, 1947. C. W. EARP 2,433,350

SUPERHETERODYNE RADIO RECEIVER HAVING coMPENsATING MEANS EDR FREQUENCYDRIFT oF THE RECEIVED CARRIER WAVE Filed July l, 1942 v Fi ' ra):04m/pm@ El. V mW/M Patented Dec. 30, :1947

SUPERHETERODYNE RADIO RECEIVER HAVING COMPENSATING MEANS FOR FREQUENCYDRIFT OF THE RECEIVED CARRIER WAVE Charles William Earp, London W. C. 2,England, assignor to International Standard Electric Corporation, NewYork, N. Y.

Application July 1, 1942, Serial No. 449,293 In Great Britain July 11,1941 (Cl. Z50-20) 7 Claims.

1 The present invention relates to superheterodyne radio receivers inwhich the frequency of the local oscillator is varied to compensate forfrequency drift of the received carrier wave by means of a voltage whichvaries in response to variations of the received carrier frequency.

According to one aspect of the invention a superheterodyne radioreceiver of the type specified 1s characterised in that the said voltageis applied to control the local oscillator through a circuit having atime constant variable under the control of the strength of the receivedwaves.

From another aspect of the invention, a superheterodyne radio receiverprovided with arrangements for eliminating the effects due to frequencydrift of the received Waves comprises a frequency discriminator orcounting device fed with intermediate frequency from the intermediatefrequency stage of the receiver and adapted to produce an output voltagedepending upon the input frequency, said voltage being fed over acircuit having a variable vtime constant to a device arranged to controla frequency determining element of the generator in such a way thatvariations in said voltage cause corresponding variations in saidfrequency determining element to vary the frequency of the oscillationsgenerated by the local generator and thereby return the intermediatefrequency to normal, the said time constant being variable under thecontrol of the strength of the received waves.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawing in which:

Fig. 1 is a block schematic diagram of a circuidt illustrating theprinciple of the invention; an

Fig; 2 is a block schematic of the circuit of a Superheterodyne receiverembodying the invention and in which an automatic frequency controloscillator, as described in my copending application SerialNumber451,292, led July 17, 1942, Patent Number 2,354,510, granted July 25,1944, is used.

Fig. 3 shows, schematically, one form of phase control circuit.

Referring to Fig. l, a superheterodyne radio receiver comprises a highfrequency stage HF, a

heterodyning or frequency changing stage FCI-1'. fed from a high'frequency oscillator LO, and an intermediate frequency amplifier stageIFA fed from the output of frequency changing stage FCH, and a lowfrequency detector DET Auto# matic frequency control arrangements forvary- LO to compensate for frequency drift in the received signalscomprise a frequency discriminator or counter DI' adapted adapted toproduce in its output circuit a voltage depending upon the inputfrequency and fed with intermediate frequency energy from theintermediate frequency stage IFA. The output voltage from the frequencydiscriminator DI is fed to a frequency control tube FC which controls inany known manner, as for example, by the Miller effect, the frequency ofthe oscillator LO. Fig. 3 shows one form of Miller effect circuit, inwhich the input capacitance of valve 26,5hun'tedbyinput resistance 21,is varied as the bias from the discriminator changes, which latter biasalters the valve transconductance. Details of such circuit are given onpage 182 of Radiotron Designers Handbook, third edition, issued byAmalgamated Wireless Valve Co. Pty. Ltd., 47 York Street, Sydney,Australia. In accordance with the invention this voltage from thefrequency discriminator DI is fed to the frequency control tube FC overa circuit having a variable time constant and comprises a condenser Cand a vari able series resistance R whose value is made to depend uponthe strength of the intermediate frequency signals. For this purpose thevariable resistance R comprises a four-element rectier bridge across onediagonal of which a biasing bat-y tery B is connected in series with oneWinding of a transformer T in such sense that the four rectier elementsof the bridge are made non-conducting. The output voltage from thefrequency discriminator DI is connected across the other diagonal of thebridge R. When the rectier bridge is non-conducting the time constant ofthe variable resistance and condenser RC circuit is infinity, or atleast very large, that is, when the output from the intermediatefrequency stage is small.

A portion of the output from the intermediate frequency stage IFA isintroduced to the primary winding of `transformer T and if sufcientlystrong overcomes t-hebiasjon the rectiers due to the batteryk B, andpasses rectified current through the battery, and through the rectiers.In thisv condition, the rectier impedance may be reduced to a low value,the series impedance along the automatic frequency control line isreduced, and the time constant ofthe line becomes short. In the presenceof ai signal, therefore, the automatic control of frequency is operativeand quick. If, now, the signal is interrupted or quickly fades the timeconstant of the variable resistance and ing the frequency of thehighfrequency oscillator condenser RC circuit becomes very large and thefrequency control voltage across the condenser C is maintained.

Referring now to Fig. 2 which shows a superheterodyne receiver embodyingthe invention and incorporating an automatic frequency controloscillator as described and claimed in said application No. 451,292, theoscillation generating arrangement and the principle on which it worksare fully described in my application Serial Number 460,578, filedOctober 2, 1942, Patent Number 2,358,152, granted September l2, 1944,

In Fig. 2 like parts are given the same designations as in Fig. 1.

A stable oscillator O of frequency F is connect` ed directly to amodulator M2, and via a phase controlling thermionic valve PC tomodulator Ml. From the output of MI filter FI selects a band of signalfrequencies of mean frequency (F-l-f) or (F-f) which signals are passedto M2. From the output of M2 filter F2 selects a frequency signal bandof mean frequency f, which is passed to MI At some point in the looppath MI-M2-Mi there is inserted a delay network DL or, alternatively,the required delay may be produced in any of the components of thecircuit, for instance, the lters.

The circuit as described up to this point produces oscillations offrequency f in the path M2 to MI, and oscillation of frequency (F-I-f)or (F-f) in the path MI to M2. Now, a phase shift produced by the phasecontrol tube PC, produces a proportional change in the frequency f. Thisfrequency f signal is now fed to a frequency discriminator D, orfrequency counter, the output voltage of which exactly matches the phasecontrol voltage necessary to maintain the frequency applied to thediscriminator.

If, now, an additional frequency control voltage is introduced from ainexternal source, the frequency of the oscillations speeds up or slowsdown, according to the sense of the control voltage, until the controlvoltage is removed.

The frequency for controlling the oscillation circuit is obtained fromthe output of either filter FI or F2. Likewise the output of eitherfilter may be used for any desired purpose, for example, as the localoscillation of the superheterodyne receiver as in the present example inwhich the output from filter FI is fed to the frequencir changer stageFCH. In Fig. 2 the source of control voltage somprises a voltagegenerated in response to the deviation of the intermediate frequencyfrom normal. The variation in the frequency of the locally generatedoscillation is controlled by the frequency discriminator D which is fedwith a frequency signal derived from the output of lter F2, i. e. offrequency f, and the voltage depending upon the frequency drift of thereceived frequency is generated by frequency discriminator DI fed fromthe output of the intermediate frequency amplifier stage IFA. These twovoltages are applied in series to the phase control valve PC. The timeconstant of the circuit between the frequency discriminator D! and thephase control valve PC is not fixed but depends upon the strength of theintermediate frequency signal as already described in reference to Fig.l.

Whilst a particular embodiment of the invention has been described byway of example others Will occur to those skilled in the art and whichfall within the scope of the appended claims, e. g. other types ofoscillation generators .may be employed, and other forms of variableresstances lil) may be employed in the variable time constant circuit.

What is claimed is:

l. A superheterodyne radio receiver comprising a detector and modulatorcircuit, a source of oscillations associated therewith, an intermediatefrequency amplifier connected in sequence to said circuit, means forvarying the frequency of said oscillations in accordance with variationsin the frequency of the received signal carrier wave and therebycompensating for said variations, and means connected so as to restrictthe operation of said compensating means to periods when a signal of atleast minimum strength sufficient to give intelligible signals at theoutput of said receiver is being received, including control meansconnected between the signal path and said frequency varying means foractuating the latter means, said control means having a long timeconstant when the signal is below said minimum strength and a short timeconstant when said signal is above said strength, whereby the control ismaintained for sudden shifts of signal strength below said minimumstrength.

2. A receiver as set forth in claim 1, in which the control meansincludes a rectifying device, means for applying a frequency-varyingpotential to the rectifying device, means for applying a blockingpotential to the rectifying device and means for applying to the latterdevice an unblocking potential corresponding to the signal strength, thearrangement being Such that the frequency-varying potential passesthrough the rectifying device when the unblocking potential has a valuehigher than that of the blocking potential.

3. A superheterodyne radio receiver comprising a detector and modulatorcircuit, a source of oscillations associated therewith, an intermediatefrequency amplifier connected in sequence to said circuit, means forvarying the frequency of said oscillations in accordance with variationsin the frequency of the received signal carrier wave to compensate forsaid variations, and means for restricting the operation of saidcompensating means to periods when a signal of at least minimum strengthsufficient to give intelligible signals at the output of said receiveris being received, comprising a signal-responsive discriminator circuit,a rectifying device including a rectifier in series and a shuntcondenser connected to the discriminator circuit output, means forapplying a blocking potential to the rectifying device, and means forapplying to the latter device an unblocking potential corresponding tothe strength of a received signal, the rectifying device output beingconnected to and controlling said means for varying the oscillationfrequency.

4. A receiver as set forth in claim 3, in which the means for applyingunblccking potential comprises a circuit connection between the outputof the intermediate frequency ampliiier and the rectifying device.

5. A superheterodyne radio receiver comprising a detector and modulatorcircuit, a source of oscillations associated therewith, an intermediatefrequency amplifier connected in sequence to said circuit, means forvarying the frequency of said oscillations in accordance with variationsin the frequency of the received carrier wave to compensate for saidvariations, and means for varying the effectiveness of the operation ofsaid frequency varying compensating means, comprising a firstsignal-responsive frequency variation discriminator and a secondfrequency variation discriminator connected to said source ofoscillations and in series with said rst discriminator, said controlmeans including a unit having a variable time constant connected to theoutputs of said discriminators.

6. A receiver as set forth in claim 5, in which said unit comprises aphase control tube circuit and a capacitor connected across the input tothe latter circuit, one side of the capacitor being connected to theoutput from the second discriminator and the other side to the outputfrom the rst discriminator.

7. A receiver as set forth in claim 5, in which said unit comprises aphase control tube circuit, a capacitor connected across the input tothe latter circuit, a rectifier bridge, one side of the capacitor beingconnected to the output from the second discriminator, and the otherside to the output from the first discrminator through said rectifierbridge, means for applying blocking po- REFERENCES CITED The followingreferences are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,165,596 White July 11, 19392,266,065 Muller Dec. 16, 1940 2,199,350 Shofstall Apr. 30, 19402,163,234 Case June 20, 1939 2,180,816 Miller NOV. 21, 1939

